Your power requirements may be completely different to what most people have, so before you buy a kit, make sure it will be adequate for your needs. Your solar panel must be able to produce at least as much power as you will draw, your battery must be able to store that much power, your charge controller must be able to handle that much power safely and your inverter must be rated high enough to safely deliver it to your device.

The way to figure out your needs is simply to check the stated input of every device you are likely to use and add them up. This should give you the maximum amount of power you are ever going to draw at one time. And then add the stated power draw of your inverter, then finally, add another 25% of that total, just for safety. You don't want to be running any of your hardware at more than 75% of its capacity, as it can increase the risk of fire and shorten the life of the unit. Take your time with this calculation, because if you get it wrong, you could (in the worst case) end up having to add another solar panel and battery and replace your charge controller and inverter, in order to meet those needs. That's practically a whole new system!

Finally, you should also take account of the type of weather you are likely to encounter in the place where you intend to be. If it's often cloudy, you will need to increase your solar panel output accordingly. Cold weather can affect battery performance. Etc..

Note

Please understand that this is a very simplistic overview for the purpose of clarifying concepts to those who are new to the whole van-life-and-solar-panel game. If you know more about your system and power needs, then you know it can get more involved.